For a number of electronic devices, high-aspect ratio semiconducting regions, e.g. diodes, extending through the thickness of a semiconductor body are desirable.
Direct diffusion of an impurity through the thickness of a semiconductor body from the top surface to the bottom surface through solid material requires diffusion times that are longer than desirable from a productivity standpoint. The semiconducting regions produced by direct diffusion through the solid semiconductor material generally spread further laterally into the interior of the body, away from a path straight through the body, than is desired. The junctions produced between the diffused regions and the body by direct diffusion are generally irregular, e.g. hour-glass shaped, when viewed in the cross-section of the body when the impurity has been diffused from both top and bottom surfaces. These undesirable characteristics of directly diffused through-thickness semiconductor regions become more pronounced as the thickness of the body is increased to provide added strength as the surface dimensions are increased to provide more area for active device fabrication.
Methods are available for enhancing the production of through-thickness regions in semiconductor bodies. One method, such as is described in U.S. Pat. No. 4,227,942, involves the use of a chemical etch to expose areas interior to and beneath the surface of the body. These etchants are generally slow acting, produce holes whose dimensions are sensitive to thickness variations unless special procedures are followed, and are sensitive to crystallographic orientations within the body, i.e., the etchants are anisotropic. Since the etchants are anisotropic, the holes produced are typically in the form of truncated tetrahedrons. Hour-glass-shaped openings through a semiconductor wafer having semiconducting regions of opposite type lining each opening are shown in "Isolated Power Feed-Thru Holes" by R. C. Joy and W. J. Nestork (IBM Technical Disclosure Bulletin, Vol. 16, No. 11, April 1974).
Other methods, such as the one described in U.S. Pat. No. 4,137,100, involve the formation of excavations, i.e., pits or trenches, in the substrate to allow the diffusion processes to commence from levels below the surface of the body. If a laser beam is used to form the excavations, either before or after the impurity is applied, the diffusion time and lateral spreading are decreased, but at the expense of damage, in the form of dislocations or diffusion pipes, to the semiconductor body.
Two additional methods of producing through-thickness diodes are thermal gradient zone melting (TGZM) and electromigration. U.S. Pat. Nos. 3,899,361--Cline and Anthony and 3,901,736--Anthony and Cline are representative of inventions employing TGZM in the production of diodes. In these methods metal-rich droplets are migrated from a location on one face of a solid body of semiconductor material (e.g. a silicon wafer) into or through that body leaving a solid plug of recrystallized semiconducting material in its wake. The movement of a metal-rich droplet through a body of semiconducting material to produce a similar recrystallized plug by the application of an electrical potential gradient (i.e. electromigration) is described in U.S. Pat. No. 4,377,423--Anthony. These modes of processing are referred to herein as "bulk migration".
U.S. '423, in addition to describing bulk migration, also discloses the use of electromigration for effecting the migration of a liquid metal droplet along a thin film of semiconducting material supported on, but not part of, a non-conducting substrate. It is pointed out therein that thin film electromigration velocities are 100 to 1000 times faster than bulk electromigration rates.
All of U.S. '361, '735 and '423 are incorporated by reference.
A limitation of bulk migration techniques is that as the size of the recrystallized semiconductor regions decreases below about 10 mils, the practice of TGZM and electromigration become increasingly difficult and complex.
As used herein, unless otherwise specified, the term "cylindrical" is used to described the surface traced by a straight line moving parallel to a fixed straight line and intersecting a circle. When dealing with holes with length-to-diameter ratios greater than 20, the walls of the hole tend to have some taper with the entrance diameter of the hole being slightly larger than the exit diameter of the hole. As used herein and in the appended claims, the term "hole" is considered equivalent to the term "bore".